1. Field of the Invention
The present invention generally relates to a reaction chamber for a chemical vapor deposition apparatus, and more particularly, to a reaction chamber for a chemical vapor deposition apparatus improved so that a uniform and accurate film can be grown stably for every wafer. The present invention also relates to a chemical vapor deposition apparatus using such a reaction chamber.
2. Description of the Background Art
A SiO.sub.2 film is conventionally used as an interlayer insulation film of VLSI semiconductor devices such as 4M bit dynamic RAMs. In accordance with reduction of size of VLSI semiconductor devices, there has been progress in the technology of forming a PSG film and a BPSG film by doping lightly phosphorus and, phosphorus and boron. The purpose of doping lightly phosphorous and boron is to facilitate softening of an interlayer insulation film by heat to allow planarization of the surface thereof,
Referring to FIG. 1A, a BPSG film 101 including B and P is formed on a substrate 100 using SiH.sub.4 and O.sub.2 by CVD. When thermal treatment of high temperature is applied to BPSG film 101, interlayer insulation film 101 is softened and the surface thereof is planarized.
This method of forming an interlayer insulation film using SiH.sub.4 and O.sub.2 gas has a problem that a void (a bubble) 102 remains in the valley of an underlying pattern 100a even if a high temperature process is carried out after film deposition because the grown film results in an overhang configuration due to increase in integration density (i.e. when the distance between patterns is reduced), as shown in FIG. 2A and 2B.
In order to solve such a problem, a method and an apparatus have been developed for an interlayer insulation film using tetraethoxy silane (abbreviated as TEOS hereinafter) which is an alcoholate type liquid source.
According to a method of forming an interlayer insulation film by a CVD method using TEOS-O.sub.3, the configuration after film deposition will result in a flow configuration without an overhang, as shown in FIG. 3A and 3B. Therefore, no void (bubble) will remain in the valley of underlying pattern 100a after a treatment of high temperature even if the integration density is increased to result in an interlayer insulation film 101 having the surface planarized and of good film quality, as shown in FIG. 3B.
FIG. 4 is a plan view schematically showing a chemical vapor deposition apparatus using TEOS, and FIG. 5 is a sectional view thereof. In general, the manufacturing process of semiconductor devices is carried out in a container called a cassette in which a plurality of wafers are inserted. The apparatus shown in FIGS. 4 and 5 is one that processes wafers in the unit of cassettes. The apparatus uses three types of liquid sources including TEOS as an alcoholate type liquid source, trimethyl-ortho-phosphate (abbreviated as TMPO hereinafter) for doping and triethyl borate (abbreviated as TEB hereinafter), and O.sub.3 gas and N.sub.2 gas are used as a reaction gas and a carrier gas, respectively.
Referring to FIGS. 4 and 5, K1 and K2 are stages for mounting actual-product cassettes which are actually subjected to processes. K3 and K4 are stages for loading empty cassettes in which wafers subjected to a film growth process are accommodated. A belt type roller 5 serves to draw out or insert wafers one by one from a cassette mounted on cassette stages K1-K4. A wafer handling robot 6 is shown. A plurality of heating stages 7 are connected as a belt. In this conventional example, 18 plates (heating stages) are connected. By a driving motor (not shown), a drive drum 8 is rotated, whereby heating stage 7 moves towards K3 (K4) from K1 (K2). A stage heater 9 is provided under heating stage 7 to heat heating stage 7. A gas supplying head 10 is provided above heating stage 7. Gas supplying head 10 is covered with an exhaust cover 11.
Bubbling material tanks 12a, 12b, and 12c contain liquid TEOS, liquid TMPO, and liquid TEB, respectively. Bubbling material tanks 12a, 12b and 12c are heated and maintained at a temperature by a heater (not shown), a thermocouple (not shown) measuring the temperature of the liquid, and a temperature controller (not shown). Bubbling material tanks 12a, 12b and 12c have the flow of N.sub.2 gas controlled and supplied via mass flow meters 13a, 13b, and 13c, respectively. Respective pipes from bubbling material tanks 12a, 12b, and 12c are combined to be connected to gas supplying head 10.
The operation thereof will be described hereinafter.
An actual-product cassette is mounted on cassette stage K1/K2. An empty cassette for accommodating a wafer that will be subjected to a growth film process is mounted on cassette stage K3/K4.
A wafer 14 is drawn out one by one from the cassette mounted on cassette stage K1 by belt type roller 5. A Bernoulli chuck 6a of wafer handling robot 6 draws up each wafer with the surface thereof upwards which is conveyed onto heating stage 7. Bernoulli chuck 6a draws wafer 14 by adsorption without coming into contact with the surface of the wafer by creating negative pressure by blowing out N.sub.2 gas as shown in FIG. 6. Similarly, wafer 14 mounted on the cassette on cassette stage K2 is conveyed to heat stage 7. The heat stage 7 having two wafers 14 loaded moves in the direction towards cassette stage K3 (K4) from cassette stage K1 (K2) at a constant speed by the rotation of drive drum 8. Wafers 14 are heated by stage heater 9 and are subjected to film growth by passing under gas supplying head 10. Gas supplying head 10 is covered with exhaust cover 11. There is a space between gas supplying head 10 and exhaust cover 11. By a duct 16 connected to a discharge fan 15, the interior of exhaust cover 11 is maintained under negative pressure to prevent the reaction gas from flowing out. The wafer having a film grown by passing under gas supplying head 10 moves towards cassette stage K3 to be accommodated in a cassette mounted on cassette stage K3 or K4 by wafer handling robot 6 and belt type roller 5.
A plurality of heat stages 7 are connected as a belt. Therefore, wafer 14 can be mounted on heat stage 7 continuously by belt type roller 5 and wafer handling robot 6 as heating stage 7 is moved, resulting in a continuous film growth process of large quantity.
Reaction gas set forth in the following is supplied to gas supplying head 10. Among the three types of liquid sources, liquid TEOS will be described. N.sub.2 gas is measured accurately by a mass flow meter 13a to be supplied into bubbling material tank 12a. Liquid TEOS maintained at a certain temperature is contained in bubbling material tank 12a. By bubbling N.sub.2 gas in bubbling material tank 12a, N.sub.2 gas is generated including TEOS gas by an amount corresponding to vapor pressure in accordance with the temperature of liquid TEOS. The N.sub.2 gas including TEOS gas is sent to gas supplying head 10 by a pipe 17. Pipe 17 is heated so that the vaporized TEOS is not re-liquefied (not shown).
The other liquid sources TMPO and TEB are similarly vaporized by bubbling and sent to gas supplying head 10.
O.sub.2 gas measured accurately by a mass flow meter 19 is supplied to an ozone generator 18. A portion of O.sub.2 gas is converted into O.sub.3 by ozone generator 18, whereby the same is sent to gas supplying head 10. Thus, gas supplying head 11 is supplied with N.sub.2 gas including TEOS, TMPO, and TEB, and O.sub.2 gas including O.sub.3 which are all mixed. The mixed gas is heated and blown onto wafer 14 passing under 9as supplying head 10. A thin film is formed by chemical vapor deposition on wafer 14. The heated temperature of wafer 14 is approximately 350.degree.-450.degree. C.
A conventional chemical vapor deposition apparatus using TEOS 9as implemented as described above had various problems in film controllability, stability, uniformity, cost, space, processing capability, and maintenance, which will be described hereinafter.
A problem in conveying a wafer will first be described.
(1) A conventional apparatus has a plurality of heating stages 7 connected as a belt as shown in FIG. 5. This structure causes wafer 14 to be continuously provided under gas supplying head 10 to be subjected to a film growth process sequentially. It was therefore impossible to grow a film under different conditions for each wafer. The two wafers 14 mounted on heating stage 7 could also be processed by only the same condition. The quality of thickness of a film could not be controlled by changing the conditions during the film growth.
(2) There is only one cassette stage K1 or K2 in each lane for mounting a cassette. Therefore, if there is some time period until the next cassette is loaded on cassette stage K1 or K2 after the process of a wafer, no wafer is inserted during that period to result in a loss. Because there is no cassette stage in a conventional apparatus for accommodating a monitor wafer in order to check the state of film growth periodically, the operator had to insert a monitor wafer periodically. There was also a problem of poor workability because the stage (K1, K2) for inserting a cassette and the cassette stage (K3, K4) for accommodating a film deposition processed wafer are not located in close proximity.
(3) Because the wafer is conveyed by a belt type roller and a wafer handling robot using a Bernoulli chuck method, contamination at the bottom of a wafer due to the rubber belt and lifting up foreign particles by the gas blow out occurred.
(4) Because the direction and positioning of the orientation flat of a wafer is not carried out in a conventional apparatus, the position of a wafer on the heating stage was variable, leading to an unstable film deposition condition.
There were also problems in the reaction chamber of a conventional apparatus as set forth in the following.
(1) Referring to FIG. 5, the reaction portion is not a closed structure. This means that there is spacing between heating stage 7 and exhaust cover 11. Therefore, a change in the exhaust status will cause a change in the state of the flow-in of external air, which in turn changes the gas flow in the reaction portion. This results in an unstable film deposition condition.
(2) In a conventional apparatus, the area on heating stage 7 where a wafer is not placed has an unrequired film applied thereon which is stacked into multiple layers. This multilayer will result in a thick film which is eventually peeled off. Reaction particles in the chamber will easily adhere to the peeled off film which is reduced in temperature. Such reaction particles will adhere to a wafer, causing decrease in yield. Furthermore, reaction particles adhering to the stage can not be removed just by wiping. The entire stage must be removed to have the reaction particles removed by applying wet etching such as with hydrofluoric acid. Therefore, there was a problem of poor maintenance.
(3) Heating stage 7 is made of stainless steel or nickel alloy due to the fact that it is subjected to wet etching with hydrofluoric acid. Stainless steel and nickel alloy have poor thermal conductivity, making it difficult to heat uniformly the wafer.
(4) Wafer 14 is loaded on heating stage 7 and moves in only one direction. Even if the gas flow supplied from gas supplying head 10 is not uniform, it can not be corrected. This resulted in a problem that the thickness distribution of the film formed on the wafer is not constant.
(5) Exhaust cover 11 covers only gas supplying head 10 and does not serve to control the flow of gas. Therefore, channeling and biased current of exhaust gas occurred, resulting in unstable film deposition conditions. There was also a problem that reaction gas flows into the area where heater 9 is located through the gap between adjacent heating stages connected as a belt, resulting in contamination of heater 9.
(6) Because the temperature of exhaust cover 11 is not controlled, there is a mixed portion of a low temperature portion and a high temperature portion. Therefore, a great amount of reaction products and reaction film will adhere to exhaust cover 11, leading to a problem that removal thereof is difficult.
(7) The supply opening of gas supplying head 10 has its diameter reduced in order to maintain constant the speed of blow out from each opening. Therefore, the flow rate of gas is high and the film at the surface of a wafer of the region corresponding to the position of each opening is thickly formed.
(8) In a conventional apparatus, TEOS type reaction gas and O.sub.3 gas are mixed together in the cavity within gas supplying head 10. Because these gases were mixed only by relative diffusion, there was a possibility of insufficient mixing, resulting in a non-uniform film thickness.
(9) Gas supplying head 10 is influenced by the temperature to maintain the heat of heating stage 7, supply gas, exhaust gas, and exhaust cover 11, so that the inside wall could not be maintained at a constant temperature. Therefore, the reaction state of the TEOS type reaction gas and the O.sub.3 gas varied. This resulted in a problem that there is deviation in the film thickness and the dopant concentration for each wafer.
The following problems were also encountered in the reaction gas supplying portion of a conventional apparatus.
(1) The amount of TEOS gas in the N.sub.2 gas corresponds to the amount of vapor pressure according to the temperature of the bubbled liquid TEOS. However, there was a great change in the vapor pressure of TEOS in response to a slight change in temperature, as shown in FIG. 7. For example, if a liquid temperature of 60.degree. C. is raised by 5.degree. C., the vapor pressure of TEOS is multiplied by approximately 1.3, which in turn causes the TEOS concentration in the N.sub.2 gas to be multiplied by approximately 1.3, resulting in a great difference in film thickness.
Although the temperature of liquid TEOS in bubbling material tank 12a is controlled by a thermal couple and a temperature controller, reduction in liquid temperature occurs due to latent heat of vaporization at the time of vaporization because the carrier gas is passed through the liquid material. The system including bubbling material tank 12a, the heater and liquid TEOS is relatively great in size. Therefore, the controllability of temperature is low and the liquid can not be immediately restored from a reduced state. It was difficult to control accurately the temperature.
FIG. 8 shows the change in liquid temperature and vapor pressure according to time elapse starting from the initiation of bubbling until the end of film deposition. It is apparent from FIG. 8 that the temperature is not controlled along the target temperature, and there is a great deviation in vapor pressure. Therefore, stability in film thickness can not be expected. Such a problem occurs in the case of TMPO and TEB, leading to a problem that the film thickness and the dopant concentration are different for each wafer.
(2) Because the number of bubbling material tanks are required corresponding to the number of types of materials in a conventional apparatus, there was the problem that the cost is high and a large space is required.